Iron valence in skutterudites: Transport and magnetic properties ofCo1−xFexSb3

Abstract

Iron in the binary skutterudite compound ${\mathrm{CoSb}}_{3}$ has a dramatic effect on transport and magnetic properties comparable to that of ``rattling'' guest atoms in filled skutterudite compounds. We have measured thermal conductivity, thermopower, electrical resistivity, Hall effect, x-ray diffraction, and magnetic susceptibility on a series of iron-doped compounds ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Sb}}_{3}$ with $x=0,$ 0.005, 0.01, 0.02, 0.05, and 0.10 prepared by induction melting and annealing. Our results show that the thermal conductivity is greatly reduced with iron doping. We observe that iron doping causes only a small rate of change in carrier concentration, \ensuremath{\sim}0.03 holes/Fe atom for samples with $x>~1\mathrm{at}.%,$ even though iron has one fewer electron in its outer shell (eight) than cobalt (nine). The room temperature thermopower is reduced with increasing iron content, and we observe phonon drag effects at low temperature. Our magnetization measurements show the development of a paramagnetic moment approaching $1.73{\ensuremath{\mu}}_{B}/\mathrm{Fe}$ atom, consistent with trivalent iron in a low-spin ${d}^{5}$ electron configuration. The subtle role of iron in creating lattice defects in ${\mathrm{CoSb}}_{3}$ that are responsible for the dramatic thermal conductivity reduction will also be discussed.